1. Field of the Invention
The present invention relates to a method of producing a crystalline silicon semiconductor film such as a polycrystal silicon film, a single crystal silicon film, and a fine crystal silicon film. The crystalline silicon films produced by using the present invention are used for various semiconductor devices.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as TFT) using a thin film semiconductor is known. This transistor is constructed by using a thin film semiconductor, especially a silicon semiconductor film formed on a substrate. TFTs are used for various kinds of integrated circuits. Especially, they have attracted attention as switching elements provided for respective picture elements of an active matrix type liquid crystal display device, or driver elements formed on a peripheral circuit portion.
As a silicon film used for the TFT, it is simple to use an amorphous silicon film. However, it has a problem that the electric characteristics thereof are much lower than those of a single crystal semiconductor used for a semiconductor integrated circuit. Thus, it has been applied only for limited use in switching elements of an active matrix circuit and the like. The characteristics of the TFT can be improved by using a crystalline silicon thin film. The crystalline silicon other than the single crystal silicon is referred to as polycrystal silicon, polysilicon, fine crystal silicon or the like. Such a crystalline silicon film can be obtained by first forming an amorphous silicon film, and then crystallized by heating (thermal annealing). This method is referred to as a solid phase growth method since the amorphous state is transformed into the crystal state while keeping the solid state.
However, the solid phase growth of silicon requires a heating temperature of not less than 600xc2x0 C. and a time of not less than 10 hours. Thus, there is a problem that it is difficult to use a cheap glass substrate as a substrate. For example, Corning 7059 glass used for an active type liquid crystal device has the glass distortion point of 593xc2x0 C., so that a difficulty arises in performing thermal annealing of not less than 600xc2x0 C., considering the enlargement of the area of a substrate.
According to the study of the present inventor(s) to solve such problems, it has been found that when a small amount of element such as nickel or palladium, or lead is deposited on the surface of an amorphous silicon film and then the film is heated, crystallization can be realized at 550xc2x0 C. and in a processing time of about four hours.
A small amount of element (catalytic element for accelerating crystallization) can be introduced by depositing a coating film of the catalytic element or its compound by sputtering. However, if a large amount of the above-mentioned element is present in a semiconductor, the reliability and electrical stability of a device using such a semiconductor are damaged, which is not preferable. If a film is formed by sputtering, it has been difficult to precisely control the amount, that is, the thickness. Also, it has been further difficult to obtain a film having a uniform thickness on a substrate. Thus, the characteristics of the thus obtained semiconductor devices have not been uniform.
Further, when a film is formed by the sputtering, an amorphous silicon film is considerably damaged by the shock of sputtering, so that the characteristics of the thus obtained semiconductor devices are not necessarily satisfactory.
There is also a method of forming a coating film by means such as spin coating instead of the sputtering. However, the spin coating method presents the difficulty in obtaining a uniform coating film. For example, in a rectangular substrate such as in a liquid crystal display, a solution is apt to collect at corner so that the film thickness has not been uniform. Further, when the solvent drys to produce a coating film of a catalytic element compound, due to the unevenness of drying or generation of crystal nuclei, the film thickness becomes uneven, which causes uneven characteristics in semiconductor devices.
In the production of a crystalline thin film silicon semiconductor by heat treatment at a temperature lower than that required for a normal solid phase growth method using a catalytic element, an object of the present invention is to satisfy the requirements of (1) enabling the control of the trace of the catalytic element, and (2) enabling the uniform introduction of the catalytic element. Further, it is also an object of the invention to (3) improve productivity when the catalytic element is introduced.
In order to attain the above objects, according to the present invention, a vapor or a gas having a catalytic element is directly or indirectly adsorbed on the surface of an amorphous silicon film, and low temperature crystallization is carried out using the adsorbed catalytic element. The above structure of the invention has the following basic features.
(a) The concentration of a catalytic element in the amorphous silicon film is determined by the amount of the catalytic element adsorbed on the surface. The amount of the catalytic element adsorbed on the surface is set based on the ratio between an adsorbing speed of the catalytic element onto the surface and a separating speed from the surface. In case that the substrate temperature and total pressure are constant, the amount can be determined uniquely by a chemical potential of a vapor or a gas containing the catalytic element in vapor phase, (by a partial pressure in this invention).
(b) During the adsorbing step onto the surface, an extremely very uniform coating film is formed on the surface. By controlling a partial pressure of a vapor or a gas containing the catalytic element, it is possible to form three kinds of adsorbing layers, that is, a complete monomolecular adsorption layer (covering rate=1), a monomolecular adsorption layer with the covering rate of less than 1, and a multilayer absorption layer consisting of a plurality of molecular layers. Especially, in the region of the complete monomolecular adsorption layer, a wide plateau region is obtained with respect to a time change and a small partial pressure change, so that the control property is very high.
(c) Since only the adsorbing phenomenon is used, a film containing the catalytic element can be formed generally at a remarkably low energy, and the amorphous silicon film receives no damage as compared with other sputtering or evaporation method.